S32205 is the tighter, higher-alloyed modern specification for 2205 duplex stainless steel. It can be dual-certified as the original S31803, but the reverse is not true. In practice, S32205 guarantees the higher end of the composition range and so a higher pitting corrosion resistance.

Background

Duplex stainless steels account for more than 80% of the consumption of duplex, super duplex and hyper duplex grades. Developed in the 1930s for paper and pulp manufacture, duplex alloys are based around a 22% Cr composition and the mixed austenitic:ferritic microstructure that delivers their mechanical properties.

Compared with the generic 304/316 austenitic stainless steels, the family of duplex grades typically has twice the strength and a significant uplift in corrosion resistance.

Why two designations exist

Increasing chromium increases pitting corrosion resistance, but the Pitting Resistance Equivalent Number (PREN) that infers an alloy's pitting resistance also includes other elements. That subtlety explains how the difference between UNS S31803 and UNS S32205 developed.

The initial duplex specification was captured as UNS S31803. However, leading manufacturers were consistently producing the grade to the upper end of the allowable specification, to maximise corrosion performance. This was aided by the AOD steelmaking process, which allowed tighter control of composition and influenced the level of nitrogen additions rather than leaving nitrogen as a background element. The highest-performing duplex therefore sought to maximise chromium (Cr), molybdenum (Mo) and nitrogen (N). The difference between an alloy meeting the bottom of the specification and one hitting the top can be several points, based on the formula PREN = %Cr + 3.3 %Mo + 16 % N.

To differentiate duplex produced at the top of the composition range, a further specification was introduced: UNS S32205. Material made to the S32205 (F60) caption fully meets the S31803 (F51) caption, whereas the reverse is not true. S32205 can therefore be dual-certified as S31803.

What we stock

We stock a comprehensive range of duplex stainless steel as the preferred distribution partner of Alleima. We hold S32205 in sizes from 5/8" up to 18" diameter in round bar, with most of our stock in the Sanmac 2205 grade, which adds enhanced machinability to the alloy's other properties. We also stock a range of S32205 hollow bar from our UK warehouse, and plate up to 3" from our USA warehouse.